Fuel ethanol is currently produced from feedstocks such as cornstarch, sugar cane, and sugar beets. However, the production of ethanol from these sources cannot expand much further due to limited farmland suitable for the production of such crops and competing interests with the human and animal food chain. Finally, the use of fossil fuels, with the associated release of carbon dioxide and other products, in the conversion process is a negative environmental impact of the use of these feedstocks
The possibility of producing ethanol from cellulose-containing feedstocks such as agricultural wastes, grasses, and forestry wastes has received much attention due to the availability of large amounts of these inexpensive feedstocks, the desirability to avoid burning or landfilling cellulosic waste materials, and the cleanliness of ethanol as a fuel compared to gasoline. In addition, a byproduct of the cellulose conversion process, lignin, can be used as a fuel to power the cellulose conversion process, thereby avoiding the use of fossil fuels. Studies have shown that, taking the entire cycle into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The cellulosic feedstocks that may be used for ethanol production include (1) agricultural wastes such as corn stover, wheat straw, barley straw, oat straw, oat hulls, canola straw, and soybean stover; (2) grasses such as switch grass, miscanthus, cord grass, and reed canary grass; (3) forestry wastes such as aspen wood and sawdust; and (4) sugar processing residues such as bagasse and beet pulp.
Cellulose consists of a crystalline structure that is very resistant to breakdown, as is hemicellulose, the second most prevalent component of cellulosic feedstocks. The conversion of cellulosic fibers to ethanol requires: 1) liberating cellulose and hemicellulose from lignin or increasing the accessibility of cellulose and hemicellulose within the cellulosic feedstock to cellulase enzymes, 2) depolymerizing hemicellulose and cellulose carbohydrate polymers to free sugars, and 3) fermenting the mixed hexose and pentose sugars to ethanol.
Among well-known methods used to convert cellulose to sugars is an acid hydrolysis process involving the use of steam and acid at a temperature, acid concentration and length of time sufficient to hydrolyze the cellulose to glucose (Grethlein, 1978, J. Appl. Chem. Biotechnol. 28:296-308). The glucose is then fermented to ethanol using yeast, and the ethanol is recovered and purified by distillation.
An alternative method of cellulose hydrolysis is an acid prehydrolysis (or pre-treatment) followed by enzymatic hydrolysis. In this sequence, the cellulosic material is first pre-treated using the acid hydrolysis process described above, but at milder temperatures, acid concentration and treatment time. This pre-treatment process increases the accessibility of cellulose within the cellulosic fibers for subsequent enzymatic conversion steps, but results in little conversion of the cellulose to glucose itself. In the next step, the pre-treated feedstock is adjusted to an appropriate temperature and pH, then submitted to enzymatic conversion by cellulase enzymes.
The hydrolysis of the cellulose, whether by acid or by cellulase enzymes, is followed by the fermentation of the sugar to ethanol, which is then recovered by distillation.
The efficient conversion of cellulose from cellulosic material into sugars, and the subsequent fermentation of sugars to ethanol, is faced with a major challenge regarding commercial viability. In particular, acid prehydrolysis requires large amounts of acid. For a clean feedstock, such as washed hardwood, the sulfuric acid demand is 0.5% to 1% of the dry weight of the feedstock; for agricultural fibers, which can contain high levels of silica, salts, and alkali potassium compounds from the soil, the acid demand can be up to about 10-fold higher, reaching 5% to 7% by weight of feedstock. This adds significant cost to the process. A second drawback of using large amounts of acids in a prehydrolysis process is that the acidified feedstock must be neutralized to a pH between about 4.5 and about 5 prior to enzymatic hydrolysis with cellulase enzyme. The amount of caustic soda used to neutralize acidified feedstock is proportional to the amount of acid used to acidify the feedstock. Thus, high acid usage results in high caustic soda usage, which further increases the cost of processing cellulosic feedstock to ethanol. Furthermore, the cost of enzymatic hydrolysis is high, as cellulose remains resistant to hydrolysis despite pre-treatment, which increases the enzyme dosage required. Such increased requirement can be counteracted by increasing the hydrolysis times (90-200 hours), in turn requiring very large reactors, which again adds to the overall cost.
A method of decreasing the enzyme dosage while maintaining high levels of cellulose conversion is Simultaneous Saccharification and Fermentation (SSF). In this type of system, enzymatic hydrolysis is carried out concurrently with yeast fermentation of glucose to ethanol in a reactor vessel. During SSF, the yeast removes glucose from the reactor by fermenting it to ethanol and this decreases inhibition of the cellulase by glucose. However, the cellulase enzymes are still inhibited by ethanol. SSF is typically carried out at temperatures of 35-38° C., which is a compromise between the 50° C. optimum for cellulase and the 28° C. optimum for yeast. This intermediate temperature leads to substandard performance by both the cellulase enzymes and the yeast. Thus, the inefficient catalysis requires very long reaction times and very large reaction vessels—both of which are costly.
A method for higher volumetric productivity is disclosed in U.S. Pat. No. 5,258,293 (Lynd). This method utilizes a lignocellulosic feedstock along with microorganisms that are continuously introduced into a reaction vessel. Fluid is also continuously added from the bottom of the reaction vessel, but no mechanical agitation of the slurry occurs. As the reaction progresses, the lignocellulosic feedstock being digested tends to accumulate in a spatially non-homogenous layer while the ethanol product rises to a top layer, where it is removed. The insoluble substrate accumulates in a bottom layer and can be withdrawn from the vessel. This arrangement results in a differential retention of the fermenting substrate, which allows for increased residence time in the reactor vessel.
In another approach, disclosed in U.S. Pat. No. 5,837,506 (Lynd), ethanol is produced using an intermittently agitated, perpetually fed bioreactor. Lignocellulosic slurry and microorganisms are added to a reactor; the mixture is then agitated, either by mechanical means or by fluid recirculation, for a specific time interval, after which it is allowed to settle. Ethanol is then removed from a top portion of the reactor, additional substrate is added, and the cycle continues. In a similar method, Kleijntjens et al. (1986, Biotechnology Letters, 8:667-672) utilize an upflow reactor to ferment cellulose-containing substrate in the presence of C. thermocellum. The substrate slurry settles to form an aggregated fibre bed, which is accelerated by slow mechanical stirring. Substrate is added periodically, while liquid is continuously fed to the reactor. Ethanol product accumulates in a top layer, where it is removed from the reactor. The methods disclosed in U.S. Pat. No. 5,837,506, U.S. Pat. No. 5,258,293 and Kleijntjens et al. result in an increase in the residence time of the feedstock in the reactor vessel. However, all three methods suffer from the disadvantages of the SSF process.
U.S. Pat. No. 5,348,871; U.S. Pat. No. 5,508,183; U.S. Pat. No. 5,248,484; and U.S. Pat. No. 5,637,502 (Scott) teach a method to improve the conversion efficiency in enzymatic hydrolysis through the use of an attritor in association with an agitated reactor vessel. The agitator produces a high-shear field for size reduction of solid particles in the cellulosic feedstock, which constantly provides new surface area for the cellulase enzymes. Therefore, the reaction efficiency is increased and the enzyme requirements are decreased. However, the high shear often inactivates the enzymes. Furthermore, the cost of the attritor equipment is much greater than the savings due to the decreased enzyme dosage.
U.S. Pat. No. 5,888,806 and U.S. Pat. No. 5,733,758 (Nguyen) teach an alternative approach using a tower hydrolysis reactor comprising alternating mixed and unmixed zones, thus reducing the mixing power consumption and cost. The slurry is moved upward in plug flow through the reactor and is intermittently mixed in the mixing zones, thus preventing channeling of liquid and ensuring uniform heat and mass transfer. While the methods disclosed in U.S. Pat. No. 5,888,806 and U.S. Pat. No. 5,733,758 reduce the shearing and denaturation of the enzymes, the cost of the mixing equipment is substantial. Furthermore, the kinetic performance of the enzymes is no better than can be achieved in a batch hydrolysis mode.
At present there is much difficulty in the art to attain high conversion efficiency while maintaining lowered costs. Increasing hydrolysis times to avoid higher costs of increasing the enzyme dosage requires larger reactors, which in turn increases equipment costs. Mixing and intermittent mixing of the feedstock during hydrolysis can increase enzyme efficiency but equipment costs will again increase, and shear forces will cause enzyme denaturation. Other systems compromise the optimal enzyme activity and reduce the efficiency of the enzymes.